The present invention relates to a pyrochemical reprocessing method of spent nuclear fuel used in atomic reactor and to an induction heating system used in the pyrochemical reprocessing method.
Studies have been performed, both domestic and abroad, on recycling method to improve economic feasibility of nuclear fuel cycle, by which the spent nuclear fuel is reprocessed by pyrochemical reprocessing method using molten salt and uranium or plutonium and the spent fuel is collected and recycled.
In the pyrochemical reprocessing of oxide fuel, spent nuclear fuel is dissolved in a molten salt, and oxides of uranium or plutonium in granular state are deposited and are collected by electrolysis. Principal processes are as follows:
Chlorination dissolving process of spent fuel:
UO2+Cl2xe2x86x92UO2Cl2
PuO2+C+Cl2xe2x86x92PuCl4+CO2
Electrolysis and collecting process of uranium oxide (cathode):
UO2Cl2xe2x86x92UO2+Cl2
Precipitating and collecting process of plutonium oxide:
PuCl4+O2xe2x86x92PuO2+2Cl2
The crucible used for reprocessing is made of pyro-graphite, and it exerts action as an anode in the electrolysis and depositing process.
In the pyrochemical reprocessing method for metal fuel, spent nuclear fuel is dissolved in a molten salt. Metal uranium or metal plutonium are deposited and are collected by electrolysis. Principal processes are as follows:
Dissolving processing of spent fuel:
Uxe2x86x92U3++3exe2x88x92; Puxe2x86x92Pu3++3exe2x88x92
Electrolysis and collecting process of metal, uranium (solid cathode):
U3++3exe2x88x92xe2x86x92U
Electrolysis and collecting process of uranium oxides or plutonium oxides (liquid cathode):
U3++3exe2x88x92xe2x86x92U; Pu3++3exe2x88x92xe2x86x92Pu
(1) In the conventional heating method used for the melting of the spent nuclear fuel in the molten salt, the crucible is directly heated. As a result, the crucible acts as heat transmitting surface, and it turns to a corrosion environment at higher temperature than the melting temperature of the salt.
(2) The crucibles are directly exposed to chlorine gas, oxygen gas, etc. used in the pyrochemical reprocessing, and they make severe corrosion environment.
(3) Compared with induction current used in the conventional metal melting method, higher frequency is needed due to the difference in electric conductivity.
(4) It is generally practiced to adopt water-cooling method in order to cool down the crucible used for induction heating. However, when water is brought into contact with the molten salt by any chance, explosion may occur.
(5) In the conventional heating method, stirring means are required to evenly maintain temperature distribution of the molten salt, and this requires the designing of the entire system in more complicated structure.
(6) In the resistance heating method, the molten salt is evenly molten and evenly stirred up, and a cylindrical crucible must be generally adopted.
To solve the above problems, it is an object of the present invention to provide a method and a system, by which it is possible to improve corrosion-resistant property of the crucible and to contribute to the safety in the pyrochemical reprocessing method.
To attain the above object, the present invention provides a pyrochemical reprocessing method for spent nuclear fuel for melting the spent nuclear fuel in a molten salt placed in a crucible and by depositing the nuclear fuel, whereby the crucible is heated by induction heating, a cooling medium is supplied to cool down, and a molten salt layer is maintained by keeping balance between the heating and the cooling, and a solidified salt layer is formed on inner surface of the crucible.
Further, the present invention provides an induction heating system to be used in a pyrochemical reprocessing method for melting a spent nuclear fuel in a molten salt placed in a crucible and for depositing the nuclear fuel, wherein said induction heating system comprises means for induction heating, and cooling means for cooling by supplying a cooling medium to the crucible.